Gravity is the force of inherent natural attraction between two massive bodies. The magnitude of the gravitational force is directly related to the mass of the bodies and is inversely related to the square of the distance between centers of mass of the two attracted bodies.
Gravity is measured as acceleration, g, usually as a vertical vector component. The freefall acceleration, g, of an object near the surface of the earth is given to a first approximation by the gravitational attraction of a point with the mass of the entire earth, Me, located at the center of the earth, a distance, Re, from the surface of the earth. This nominal gravity value, g=G×Me/Re2, is about 9.8 m/s2. Thus, the freefall acceleration due to gravity near the earth's surface of an object having a small mass compared to the mass of the earth is about 9.8 m/s2. The common unit of measurement for gravity is the “Galileo” (Gal), which is a unit of acceleration defined as 1 cm/s2. One Gal generally approximates 1/1000 (10−3) of the force of gravity at the earth's surface. An instrument used to measure gravity is called a “gravimeter.”
The most accurate gravimeters are absolute gravimeters. Interferometric absolute gravimeters usually use a freefall test mass and a laser or single-frequency light beam which reflects from the freefalling test mass. The reflected light beam is combined with a reference light beam to develop interference fringes. Interference fringes are instances where the amplitude or intensity of the reflected and reference light beams add together to create increased intensity, separated by instances where the two beams cancel or create diminished intensity.
Fringes occur on a periodic basis depending upon the change in the optical path length of the reflected beam relative to the optical path length of the reference beam. One fringe occurs whenever the optical path difference between the reflected and reference beams changes by the wavelength of the light of the two beams. The path length of the reflected beam changes as it is reflected from the freefalling test mass, and that change in path length is directly related to the acceleration of the test mass caused by gravity. The fringes taken together as a set comprise a record of the distance that the freely falling body moves, and that distance is directly related to the gravity or acceleration of the freefall test mass. The use of optical fringe interferometry to measure gravity is well known. U.S. Pat. No. 5,351,122 describes an example of such a gravimeter.
A gravimeter is subject to naturally-occurring and man-made disturbances, such as seismic noise, mechanical vibrations and other physical perturbations. The disturbances cause minute changes in the path lengths of the reflected and reference light beams in an interferometric gravimeter. When the reflected and reference light beams are combined, the resulting fringes include information generated by the disturbances and not by gravity. Consequently, the accuracy of the gravity measurement suffers due to the errors introduced by the disturbances.
Natural seismic noise is a naturally-occurring physical disturbance which is particularly troublesome in interferometric gravimeters. Natural seismic noise is the natural up-and-down movement of the earth surface at an oscillatory period of about 3 to 6 seconds. The frequency of seismic noise is comparable to the typical frequency of ocean waves. Natural seismic noise typically creates vertical movement of about one micron (1μ) at the earth surface. While a one micron vertical movement of the earth surface cannot be sensed humanly, it is a very significant disturbance in an interferometric gravimeter. Typically in an interferometric gravimeter, fringes occur when the reflected and reference beam path lengths differ from one another in increments of one nanometer (1 nm). Natural seismic noise of about one micron is 1000 times greater than the typical path length difference which creates a fringe. Consequently, natural seismic noise has the potential to obscure the gravity measurement data with irrelevant and distracting fringes.
One technique used to eliminate or substantially reduce the effect of natural seismic noise in an interferometric gravimeter is to include a stabilized reflector in the path of the reflected light beam. The stabilized reflector is isolated from the effects of seismic noise by suspending it from a conventional long period isolation device. In essence, the long period isolation device functions as a spring which has a natural oscillatory period which is many times longer than that natural oscillatory period of seismic noise. With a long natural oscillatory period, the long period isolation device inertially stabilizes and isolates the stabilized reflector by disconnecting or decoupling it from the effects of seismic noise. In this manner, the reflected light beam becomes substantially unaffected by seismic noise. The reflected light beam interacts with the freefalling test mass and is also substantially unaffected by seismic noise because the freefall test mass is disconnected or decoupled from the earth while in freefall. When the reference and reflected light beams are combined, some of the effects of natural seismic noise are eliminated to achieve a more accurate gravity measurement. This technique is described in U.S. Pat. No. 5,351,122, and in “A New Generation of Absolute Gravimeters,” Metrologia, vol. 32, pp. 159-180, 1995.
Short period disturbances are difficult to suppress in an interferometric gravimeter. Short period disturbances, such as mechanical vibrations and other types of physical perturbations, are typically man-made and result from vehicles moving over the earth surface, or people or animals walking or running on the earth surface, or the operation of heavy machinery. Short period disturbances also arise from natural ambient environmental conditions, such as wind gusts which impact the gravimeter when set up in an outdoor environment or wind guests which impact trees and other nearby structures which transfer the impact forces as movement to the earth surface.
The long period isolation device provides theoretical inertial stabilization and isolation of the stabilized reflector against short period external disturbances. However, the stabilization and isolation may not be complete from a practical standpoint. A conventional long period isolation device includes electronic components and a feedback control mechanism which are intended to respond principally to long period disturbances. Consequently, the control loop response of the long period isolation device may not be fully effective in suppressing and isolating the stabilized reflector from some types of short period disturbances.
Short period disturbances have the potential to significantly impact the freefall test mass during freefall. Even though the freefall test mass is mechanically decoupled from the gravimeter and the earth during freefall, short period disturbances may impact the test mass at the instant when it is released for freefall, thereby rotating the test mass while in freefall. Rotation of the freefall test mass has the effect of changing the path length of the reflected beam path relative to the length of the reference beam path. The change in path length results from the short period disturbance which induce rotation of the freefall test mass, not from the effect of gravity. Consequently, short period disturbances which rotate the freefall test mass during freefall create anomalous fringes which introduce errors into the gravity measurement.
The inertial isolation functionality from the long period isolation device usually prevents the stabilized reflector from rotating in a similar manner or to the same degree as the freefall test mass rotates. In those circumstances where the long period isolation device is incapable of fully isolating the stabilized reflector from short period disturbances, the movement or rotation of the stabilized reflector will typically be different in extent or degree compared to that of the freefall test mass.